In cellular mobile radio telephone systems, it is fundamental that a mobile station with an established connection on a radio channel be able to maintain the established connection when moving from one cell served by one base station to another cell served by another base station. It is also desirous that a mobile station within an established connection on a radio channel be able to maintain the established connection within the same cell when that radio channel is subject to increased interference. The process by which a mobile station is able to maintain an established connection when moving from cell to cell is the handoff operation.
According to conventional cellular systems, both FDMA (frequency division multiple access) and TDMA (time division multiple access) using plural frequencies are employed for providing mobile stations within the coverage of the base station with a number of communication channels. In CDMA (code division multiple access), one wide radio frequency band that has no predetermined time slots is shared by all mobile stations. Within that frequency band, plural mobile stations may simultaneously transmit over plural channels by frequency shifting each carrier wave corresponding to a particular mobile station according to a different code. Accordingly, in FDMA, each channel is assigned to a different frequency; in TDMA, each channel is assigned to a corresponding time slot or time slots for transmission over one frequency; and in CDMA, each channel is assigned a different code. Each of these communication channels are typically unique with respect to adjacent base station coverage areas.
Cellular mobile radio telephone systems often require some type of channel use or allocation plan for determining which communication channels are to be used by mobile stations in a particular cell via a corresponding base station. For example, FDMA systems typically include a frequency reuse plan which places restrictions on the reusing of frequencies allocated to adjoining cells. The frequency reuse plan precludes adjacent cells from utilizing the same frequencies in order to avoid problems due to co-channel interference. TDMA systems typically include a frequency/time slot reuse plan which places restrictions on the reusing of combinations of frequencies and time slots allocated to adjacent cells. A plan for reuse of time slots on frequencies in a TDMA system may preclude adjacent cells from utilizing the same time slot of the same frequency in order to avoid problems due to co-channel interference.
Instead of fixed allocation of channels according to a reuse plan, some cellular systems dynamically or adaptively allocate communication channels. All of the channels become a common resource for all cells to be allocated in accordance with prevailing traffic. Nevertheless, the use of the same channels in adjoining or neighbor cells may have to be precluded in order to avoid problems due to interference.
Message signalling, such as commanding a handoff, is usually carried out through communication between the mobile station and the base station with which the mobile station has an established connection. In a known analog cellular network, the base station continuously monitors the signal strength of the call connection with the mobile station, and if the signal strength is determined to be lower than a particular threshold, a central switching control unit will initiate a handoff procedure. A target base station and an associated channel will be determined for the mobile station, and the mobile station will be instructed to tune to the selected channel of the target base station. However, this instruction is sent by the original base station, and if the mobile station has already left the coverage area of this base station or the message is inhibited by interference, the handoff will be unsuccessful, thus leading to a disconnection of the call connection.
U.S. Pat. No. 4,811,380 issued to Spear on Mar. 7, 1989, offers a proposed solution to this problem by disclosing a cellular radio telephone system with dropped call protection. The disclosed system operates in such a manner that if a handoff command sent by a base station is not received by the mobile station, the mobile station determines that the call connection has been lost. The mobile station scans the control channels in order to request a connection via a control channel associated with a base station with the highest signal strength available. This base station is most likely the target base station for the handoff, and the switch controller will request a reconnection of the call. The mobile station thereafter is assigned a traffic channel on a new frequency from the target base station. This procedure requires modifications of presently used cellular standards like TACS, IS-3 and IS-54, before it can be used in such systems.